I. Field
The present application relates generally to wireless communications, and more specifically to an apparatus and method for inclusion of the femto cells in neighbor list messages and the use of the neighbor list message in femto cell system selection.
II. Background
Wireless communication systems are widely deployed to provide various types of communication (e.g., voice, data, multimedia services, etc.) to multiple users. As the demand for high-rate and multimedia data services rapidly grows, there lies a challenge to implement efficient and robust communication systems with enhanced performance.
Generally, a wireless multiple-access communication system can simultaneously support communication for multiple wireless terminals. In such a system, each terminal can communicate with one or more base stations via transmissions on the forward and reverse links. The forward link (or downlink) refers to the communication link from the base stations to the terminals, and the reverse link (or uplink) refers to the communication link from the terminals to the base stations. This communication link can be established via a single-in-single-out (SISO), multiple-in-signal-out (MISO), or a multiple-in-multiple-out (MIMO) system.
In addition to mobile phone networks currently in place, a new class of small base stations has emerged, which may be installed in a user's home and provide indoor wireless coverage to mobile units using existing broadband Internet connections. Such personal miniature base stations are generally known as access point base stations, or, alternatively, Home Node B (HNB) or femto cells. The coverage area of a femto cell is relatively small and may be overlapped with neighboring wireless systems and other femto cells. Femto cells are also connected to the wireless communication system using an Internet Protocol based transmission system. Typically, such miniature base stations are connected to the Internet and the mobile operator's network via broadband fixed access such as via a DSL router or a cable modem. Alternatively, the femto cells may be connected to use mobile broadband access such as via HSDPA.
Neighboring wireless communication systems may employ one or more radio access technologies on one or more frequencies typically via fixed base stations, also known as macro cells. Furthermore, a system may have one radio access technology overlaying another. For example, portions of a GSM system may be overlapped with WCDMA base stations which may be overlapped with various femto cells. It may be desirable for the user equipment (UE) communicating on the macro cell to handoff to the femto cell system either when idle or while in call without dropping an active call.
In WCDMA technology a common frequency band allows simultaneous communication between a user equipment unit (UE) and plural base stations. Signals occupying the common frequency band are discriminated at the receiving station through spread spectrum CDMA waveform properties based on the use of a high speed, pseudonoise (PN) code. These high speed PN codes are used to modulate signals transmitted from the base stations and the user equipment units (UEs). Transmitter stations using different PN codes (or a PN code offset in time) produce signals that can be separately demodulated at a receiving station. The high speed PN modulation also allows the receiving station to advantageously generate a received signal from a single transmitting station by combining several distinct propagation paths of the transmitted signal. In CDMA, therefore, a user equipment unit (UE) need not switch frequency when handover of a connection is made from one cell to another. As a result, a destination cell can support a connection to a user equipment unit (UE) at the same time the origination cell continues to service the connection. Since the user equipment unit (UE) is always communicating through at least one cell during handover, there is no disruption to the call.
To facilitate such handoff, a UE must periodically search for base stations on alternate frequencies and/or alternate radio access technologies on nearby cells. A list of potential neighboring base stations is transmitted from a base station to a mobile station. This list is commonly referred to as a neighbor list message (NLM). By limiting searches to those base stations on the NLM, a UE can reduce the amount of time required for such searching as well as the battery consumption.
Aside from overlapping coverage areas, femto cells may be deployed on a dedicated frequency spectrum or may share spectrum with macro cells. In a femto cell deployment, a set of pseudo noise (PN) offsets or primary scrambling codes (PSCs) are reserved for the femto cells. In UMTS deployment, PN offsets are sometimes referred to as scrambling codes. The terms “PN offsets” and “scrambling codes” and their singular forms are used interchangeably unless noted otherwise. However, when searching is required on an alternate frequency, the time available for such searching may be limited while an active call is in session. There is therefore a need in the art for deployment of femto cell neighbor list message so that the time available for scanning alternate frequencies is minimized. The distinction between macro cell and femto cell aid UEs in terms of system selection.